1. Field of the Invention
The present invention relates to a nitride based light emitting device, and more particularly, to a nitride based light emitting device which can achieve an improvement in light emitting efficiency and reliability.
2. Discussion of the Related Art
Light emitting diodes (LEDs) are well known as a semiconductor light emitting device which converts current to light, to emit light. Since a red LED using GaAsP compound semiconductor was commercially available in 1962, it has been used, together with a GaP:N-based green LED, as a light source in electronic apparatuses including telecommunication apparatuses, for image display.
The wavelength of light emitted from such an LED depends on the semiconductor material used to fabricate the LED. This is because the wavelength of the emitted light depends on the band gap of the semiconductor material representing energy difference between valence-band electrons and conduction-band electrons.
Gallium nitride (GaN) compound semiconductor has been highlighted in the field of high-power electronic devices because it exhibits a high thermal stability and a wide band gap of 0.8 to 6.2 eV.
One of the reasons why GaN compound semiconductor has been highlighted is that it is possible to fabricate semiconductor layers capable of emitting green, blue, and white light, using GaN in combination with other elements, for example, indium (In), aluminum (Al), etc.
Thus, it is possible to adjust the wavelength of light to be emitted, using GaN in combination with other appropriate elements. Accordingly, where GaN is used, it is possible to appropriately determine the materials of a desired LED in accordance with the characteristics of the apparatus to which the LED is applied. For example, it is possible to fabricate a blue LED useful for optical recording or a white LED to replace a glow lamp.
By virtue of the above-mentioned advantages and other advantages of GaN-based LEDs, the GaN-based LED market has rapidly grown. Also, techniques associated with GaN-based electro-optic devices have rapidly developed since the GaN-based LEDs became commercially available in 1994.
The brightness and power-output of the above described GaN-based LEDs generally depend on the structure of an active layer, the extraction efficiency of light to the outside, the size of LED chips, the kind and installation angle of a mold that is required for assembling of a lamp package, fluorescent materials, etc.
Meanwhile, one of the reasons why the GaN-based semiconductors have a difficulty in growth as compared to other III-V compound semiconductors is that there are no high quality substrates, namely, wafers made of GaN, InN, AlN, or the like.
Accordingly, an LED structure is grown on a heterogeneous substrate, such as a sapphire substrate. This may cause many defects having a serious effect on the performance of the resulting LED.
FIG. 1 illustrates the basic structure of a GaN based LED. As shown in FIG. 1, an n-type GaN semiconductor layer 10 is first provided, and then, an active layer 20 having a quantum well structure is located adjacent to the n-type GaN semiconductor layer 10. In turn, a p-type GaN semiconductor layer 30 is located adjacent to the active layer 20.
As shown, the above described LED structure is grown over a substrate 40. An electrode will be formed on the LED structure in the following process. With injection of electric changes through the electrode, the resulting LED is able to emit light.
FIG. 2 is an energy band diagram of the above LED structure.
In this case, a well 21 and a barrier 22, which constitute the quantum well structure of the active layer 20, are made of GaN and InGaN, but GaN and InGaN have a large difference between their lattice constants. Therefore, with such a large difference between the lattice constants of the well 21 and the barrier 22, the active layer 20 must be affected by a serious strain that causes imperfections in solids, such as, dislocation.
Moreover, the strain is locally generated, and thus, may hinder efficient electron-hole coupling required for emitting light. Accordingly, there is a need for adjustment of the strain.
To adjust the strain, although not shown, an InGaN layer may be inserted into a part of the active layer. However, the partially inserted InGaN layer may restrict efficient confinement of electrons and holes within an active layer, thus causing a degradation in optical efficiency.